JP2541042B2 - Heat treatment method for (α + β) type titanium alloy - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for an (α + β) type titanium alloy for obtaining excellent strength, toughness and ductility.

[0002]

2. Description of the Related Art Generally, heat treatment methods for (α + β) type titanium alloys include (α + β) region heat treatment and β region heat treatment. It is well known that the characteristic of the (α + β) heat treatment is that it is composed of equiaxed α and β transformation structures and is excellent in balance of strength and ductility and fatigue characteristics.

[0003] Further, as a heat treatment for achieving particularly high strength, there is solution aging treatment. This is because the solution aging treatment, which is the first heat treatment, is heated and held in the (α + β) region below the β transformation point, and then cooled to room temperature at a high speed such as cold water.
The aging treatment, which is the second stage heat treatment, is performed at around 450 ° C to 600 ° C. For example, a heat treatment method for Ti-6Al-4V, which is a typical (α + β) type titanium alloy, is specified in American Standards AMS4911E and 4965E. According to this, after keeping at 720 ± 15 ° C. for 20 minutes, air-cooling or solution treatment as solution aging treatment is 955 ± 1
Hold at 5 ° C for 1 to 2 hours, then cool with water and subject to aging treatment of 485 to 6
After holding at 20 ° C. for 4 to 8 hours, it is a method of performing air cooling.

On the other hand, it is known that the heat treatment in the β region is characterized by having needle-like α and β transformation structures and excellent in fracture toughness and creep characteristics.

However, the β-particle size becomes coarse during heating, and needle-shaped α precipitates at the grain boundaries, resulting in a large decrease in ductility.

As a method for solving this problem, Japanese Unexamined Patent Publication No.
In Japanese Patent Laid-Open No. 3-223155, Ti-6Al-4V or Ti is used.
After heating the -6Al-4V-2Sn alloy to a β transformation point or higher and a β transformation point + 150 ° C, extrusion is performed at an extrusion ratio of 10 or higher, followed by cooling at 5 ° C / s or higher and then annealing at a temperature of 700 to 850 ° C. Therefore, it is proposed that the structure has fine β grains and that the strength, toughness, and ductility be improved.

[0007]

However, in the above-mentioned technique, a ductile structure cannot be obtained unless plastic working is performed in the β region, and it is possible to apply it to a non-uniform plastic working region such as forging. It's not something.

The present invention is intended to solve the above-mentioned problems, and high strength, high toughness and high ductility can be obtained by heat treatment in the β region without plastic working (α +
An object of the present invention is to provide a heat treatment method for β) type titanium alloy.

[0009]

In order to achieve the above object, the present inventors have examined the heating rate, temperature, time and cooling rate of solution heat treatment and the aging temperature, and as a result, the above characteristics can be achieved. I found it.

That is, the heat treatment method according to the present invention is characterized in that the alloy composition having an equiaxed structure as a pre-structure satisfies the following conditions, and when the solution aging treatment is performed on the α + β type titanium alloy, the alloy composition ( (% By weight) is the following (1) to (6)
When solution hardening aging treatment is performed on an (α + β) type titanium alloy having a composition that satisfies the condition of the formula and the balance is Ti and unavoidable impurities, the solution treatment temperature is not less than β transformation point (Tβ) and β transformation point (Tβ). ) It is a heat treatment method for an (α + β) type titanium alloy, which is heated to less than + 100 ° C at a heating rate of 2 ° C / s or more, held for 5 minutes or more, and cooled at 2 ° C / s or more.

3.0 ≦ Al ≦ 5.0 (1) 2.1 ≦ V ≦ 3.7 (2) 0.85 ≦ Fe ≦ 3.15 (3) 0.85 ≦ Mo ≦ 3.15 (4) ) 0.06 ≦ O ≦ 0.20 (5) and 80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) Further, the heat treated (α + β) type titanium alloy is further added. The aging treatment is performed at 400 to 750 ° C. for the heat treatment of the (α + β) type titanium alloy.

First, the heating temperature will be described. Since the present alloy composition has more β-stabilizing elements than Ti-6Al-4V, these β-stabilizing elements are concentrated in β before solution treatment, but are hardly contained in α. Therefore, it was found that the β transformation point of pro-eutectoid α itself is 100 ° C. or more higher than the equilibrium β transformation point of this alloy. Therefore, the present alloy is converted from β transformation point to β transformation point +100
Even if heated to ℃, the entire structure does not immediately become β structure, and α phase is transformed by diffusion from the grain boundary.
The particle size does not become coarse.

Next, the reason for limiting the heating rate will be described. If heating is performed at less than 2 ° C./s before reaching the above heating temperature, diffusion progresses during heating and the α phase becomes small, and if the β transformation point or higher is reached, β is easily formed and grain coarsening occurs and ductility decreases. br />

Further, regarding the heating time and the cooling rate, the β particle size is coarsened similarly to the case where the holding is carried out for 5 minutes or more or the cooling is carried out at 2 ° C./s or less.

Next, the reason for limiting the aging temperature will be described.
If the aging temperature is 400 ° C. or lower, the temperature is too low to increase the strength, and if it exceeds 750 ° C., the strength is increased and the strength is immediately softened. Therefore, the optimum aging temperature and 400 ° C. or more and 500 ° C. or less are set.

Finally, the reasons for limiting the alloy components in the present invention will be explained. Al: Al is alpha + beta type titanium alloy strength, sufficient strength can be obtained with less than 3% be a basic component to increase the, 3 for ductile resulting intermetallic compound exceeds 5% conversely decreases The range is 5%.

The β-stabilizing elements V, Fe and Mo have a great effect of lowering the β-transformation point and are present in large amounts in the β-transformation structure. Therefore, the addition of these causes a large difference between the entire β transformation point and the β transformation point of only the α phase before the heat treatment, and has an effect of suppressing coarsening of the β grain size in a certain temperature range.

V: If V is less than 2.1%, it becomes difficult to form an α + β type titanium alloy. Conversely, if 3.7% or more, it forms a solid solution in the α phase and the difference between the initial α phase and the overall β transformation point. On the contrary, it becomes smaller and the heating temperature range is narrowed.

Fe: Fe is one of the elements having the greatest β stability, and if it is less than 0.85%, the contribution of the β stabilizing element to the β phase is small, and the difference between the pro-eutectoid α and the overall β transformation point is small. Is small and the heating temperature range is narrow. Further, since the diffusion coefficient is very large, if it is added in an amount of 3.15% or more, the β particle size becomes coarse during solution treatment, and the ductility decreases.

Mo: Mo has a slow diffusion rate and if it is less than 0.85%, coarsens β particles during solution treatment and reduces ductility. Further, if it exceeds 3.15%, since Mo is a heavy element, the alloy density is increased.

O: O has a great effect on the strength and is 0.0
If it is less than 6%, the strength level is low. On the contrary, if it is 0.2% or more, the strength is large but the ductility is lowered.

80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) Equation (6) shows the stability of the β phase and is closely related to the strength and the β transformation point. If this value is less than 80, the β-phase β-stability is small, and the difference between the pro-eutectoid α and the overall β-transformation point is small,
The heating temperature range becomes smaller. Further, when it exceeds 130, the β-stabilizing element becomes a solid solution also in the pro-eutectoid α, and the heating temperature range is reduced.

[0023]

EXAMPLES Example 1 In Table 1, Ti-4.5Al-3V-2Fe-2 produced in the α + β region and having an equiaxed two-phase structure.
Mo-0.1O (X = 99) alloy φ15 mm round bar was used to change the heating rate, temperature, holding time, and cooling rate. The tensile properties and notch tensile properties are notched tensile strength (NTS) and Notched tensile strength divided by 0.2% PS of smooth test piece, notch yield ratio (NTS /
0.2% PS), and NTS / 0.2% PS is 1.
It is considered that the toughness is high when 0 or more. Notched tensile strength (NTS) is non-notch diameter 8.0 mm, notch bottom diameter 5.66 mm, notch shape 60 ° V, notch tip R
A test piece having a 0.1 mm stress concentration factor Kt = 5.42 is used.

As a result, when the heating rate, temperature, holding time and cooling rate do not satisfy the conditions of the present invention, the elongation is 8.
If it is less than 0% and the notch yield ratio is less than 1.0, it becomes small. Further, when aging is performed, the strength and the notch yield ratio significantly increase, although the strength decreases.

[0025]

[Table 1]

Example 2 A φ15 mm round bar manufactured by heating and rolling an α + β type titanium alloy shown in Table 2 into an α + β type titanium alloy was heated at a heating rate of 2 ° C./s, a heating temperature of 1000 ° C., and held. The tensile properties and the notch tensile properties when heat-treated at a cooling rate of 2 ° C./s for 30 seconds were investigated.

As a result, Al is less than 3% and O is 0.06.
If it is less than%, the tensile strength tends to be low. 20x
It can be seen that if (Fe) + 13x (V) + 10x (Mo) is less than 80 or more than 130, the strength is obtained, but the elongation and the notch yield ratio become small.

[0028]

[Table 2]

[0029]

As described above, according to the present invention, high strength, high toughness, and high ductility can be obtained by the β region heat treatment without plastic working.

Claims (2)

(57) [Claims] 1. The alloy composition (% by weight) below (1)
When the solution hardening aging treatment is performed on the (α + β) type titanium alloy having the composition of the following formulas (6) and the balance being Ti and unavoidable impurities, the solution hardening aging treatment temperature is not less than the β transformation point (Tβ). Below β transformation point (Tβ) + 100 ° C, 2
After heating for 5 minutes or more at a heating rate of ℃ / s or more, 2 ℃
/ S or more cooling, the heat treatment method of the (α + β) type titanium alloy. 3.0 ≦ Al ≦ 5.0 (1) 2.1 ≦ V ≦ 3.7 (2) 0.85 ≦ Fe ≦ 3.15 (3) 0.85 ≦ Mo ≦ 3.15 (4) 06 ≦ O ≦ 0.20 (5) and 80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) 2. The heat treatment according to claim 1 is performed (α + β
(Type) titanium alloy, and further aging treatment is performed at 400 to 750 ° C. (α + β) type alloy heat treatment method.